Using the Build System

This part of the guide is about using the build system, that is,
how to interact with the build system when you're developing some part
of GHC, its libraries or tools. The section aims to be comprehensive;
for a quick start, read Building/Hacking first.

Source trees and build trees

Sometimes we want to separate the build tree from the source tree.
There are a few advantages to doing this:

You can make multiple different builds from the same sources,
perhaps for testing different build settings, or for building
on different platforms.

You might want to put the source tree on a remote, backed-up,
filesystem, but keep your build tree on a local fast unbacked-up
drive (this is a configuration we use regularly at GHC HQ). It
doesn't matter if you lose the build tree: it can easily be
regenerated.

It's easy to blow away a build tree and start again, without
modifying your source tree. make maintainer-clean is usually
good for this too, but it can miss files that it doesn't know
about, or files that are remnants from older versions of GHC.

It helps to avoid mistakes whereby you edit a file that happens
to be automatically generated, instead of the original source
file (e.g. editing config.mk instead of config.mk.in). If
you only edit files in the source tree, then this can't happen.

However, if you just want to build the software once on a single
platform, then your source tree can also be your build tree, and you
can skip the rest of this section.

Windows users: so far as we know, symbolic links do not work right on MSYS at least, so we never use separate source and build trees on Windows.

A build tree is just an exact copy of the source tree, except that
every file in it is a symbolic link to the appropriate file in the
source tree. There are "standard" Unix utilities that make such
copies, so standard that they go by different names: lndir and
mkshadowdir are two (If you don't have either, the GHC source
tree contains sources for the X11 lndir check out
utils/lndir).

You need to be a bit careful, though, that any new files you create
(if you do any development work) are in the source tree, not the build
tree!

Booting, configuring, cleaning

GHC uses the autoconf tools in the standard Unixy way, described in more detail in subsequent subsections on this page:

Steps to prepare for building

We gave a quick getting-started introduction to building GHC in
Building/QuickStart, the following sections describe each step
in more detail.

In the build system, the path to the top of your build tree is
referred to as $(TOP), and we will use that convention in the
following sections.

Generate configure scripts

NOTE: if you're starting from a source distribution that you
downloaded from the GHC web site, rather than git sources, you can
skip this step. Source distributions come with configure scripts
pre-generated for you.

Change directory to $(TOP) and issue the command

$ perl boot

(Note: the $ is the prompt. You don't type that bit.) This runs
autoreconf on the various configure.ac scripts in the GHC build
tree, generating configure scripts and other miscellaneous files.

You have to re-do this step if (and only if) you ever change one of
the files that autoconf uses to generate the configure scripts,
such as configure.ac, or aclocal.m4.

Run the configure script

Run the configure script, thus:

$ ./configure <args>

configure's mission is to scurry round your computer working out
what architecture it has, what operating system, whether it has the
vfork system call, where tar is kept, whether gcc is
available, where various obscure #include files are, whether
it's a leap year, and what the systems manager had for lunch. It
communicates these snippets of information in two ways:

It translates various files such as mk/config.mk.in to
mk/config.mk, substituting for things between "@"
brackets. So, "@HaveGcc@" will be replaced by "YES" or
"NO" depending on what configure finds.
mk/config.mk is included by every Makefile (directly or
indirectly), so the configuration information is thereby
communicated to all Makefiles.

It translates mk/config.h.in to mk/config.h. The
latter is #included by various C source files, which can
thereby make use of configuration information.

configure takes some optional arguments. Use ./configure --help
to get a list of the available arguments. Here are some of
the ones you might need:

--build=<platform>

Specifies the "build platform" (see platform names).
This is usually only necessary on Windows, where you should set it
to --build=i386-unknown-mingw32.

Set the "target platform" (see platform names).
Usually not necessary.

--with-ghc=<path>

Specifies the path to an installed GHC which you would like to use.
This compiler will be used for compiling GHC-specific code (eg. GHC
itself). This option cannot be specified using build.mk
(see later), because configure needs to auto-detect the
version of GHC you're using. The default is to look for a compiler
named ghc in your $PATH.

--with-gcc=<path>

Specifies the path to the installed GCC. This compiler will be used
to compile all C files, except any generated by the installed
Haskell compiler, which will have its own idea of which C compiler
(if any) to use. The default is to use gcc. On Windows, this
should be set to the gcc that comes with MinGW, which by default
is c:/mingw/bin/gcc.

--with-ld=<path>

Specifies which ld program to use. Normally only necessary on
Windows, where you should set it to the ld that comes with MinGW,
which is usually c:/mingw/bin/ld.

--prefix

Tells the build system where you would like GHC to be eventually
installed. You don't have to install GHC in order to use it: it is
entirely possible to work on GHC and test it without ever modifying
anything outside the build tree. However, if you do want to install
GHC, then the --prefix directory is the root of the install tree.
Typically on Unix systems the default for --prefix is
/usr/local. See also Building/Installing for more details.

Build configuration

Next, you say how this build of GHC is to differ from the standard
defaults by creating a new file mk/build.mkin the build tree.
This file is the one and only file you edit in the build tree,
precisely because it says how this build differs from the source.
(Just in case your build tree does die, you might want to keep a
private directory of build.mk files, and use a symbolic link in each
build tree to point to the appropriate one.)

mk/build.mk is purely for overriding settings that are found in
mk/config.mk. You should never edit mk/config.mk directly, since
it is automatically generated from mk/config.mk.in by configure.
Neither should you edit mk/config.mk.in; just provide your settings
in mk/build.mk.

We provide a sample file, mk/build.mk.sample, which you can copy to
mk/build.mk and edit. It provides a number of pre-defined
configurations, such as a "fast build" or a "development build". Take
a look in the file itself for details.

To understand more about what you can put in mk/build.mk, read on.

Common build.mk options

The following are some common variables that you might want to set in
your mk/build.mk. For other variables that you can override,
take a look in mk/config.mk.in.

(default: NO)
Set to YES to enable profiling for GHC itself (stage 2). You'll also need to add GhcLibWays += p to make this work.

GhcDebugged

(default: NO)
Set to YES to pass -debug when building GHC (stage 2).

GhcLibWays

(default: v p)
Ways in which to build the libraries. Must contain
at least v (the vanilla way). Also contains p by default (profiling). For other
ways, see mk/ways.mk.

SplitObjs

(default: YES if supported, NO otherwise)
When set to YES, static library object files are split into smaller
pieces. This means that less of the library code needs to be linked
into the final application, which makes smaller binaries. It takes
longer to build libraries this way, though.

LAX_DEPENDENCIES

(default: NO)
When set to YES, dependencies on the ghc executable will be turned into
order-only dependencies. What this means in practice is that less needless
recompilation will be done while you are making changes to ghc's sources,
but for certain types of changes it means the build will fail.

BUILD_DOCBOOK_HTML

BUILD_DOCBOOK_PS

BUILD_DOCBOOK_PDF

(default: YES if supported, NO otherwise)
When set to YES, these build the docbook documentation (e.g. the
users guide) as HTML, PS and PDF respectively.

INTEGER_LIBRARY

By default this is set to integer-gmp, which means Integer is implemented
on top of the C GMP library. If you set it to integer-simple then a
simple, BSD-licensed Haskell implementation will be used instead.

How to make GHC build quickly

The GHC build tree is set up so that, by default, it builds a compiler
ready for installing and using. That means full optimisation, and the
build can take a long time. If you unpack your source tree and
right away say ./configure; make, expect to have to wait a while.
For hacking, you want the build to be quick - quick to build in the
first place, and quick to rebuild after making changes. Tuning your
build setup can make the difference between several hours to build
GHC, and less than an hour.

These options are added to the command line for all Haskell
compilations. We turn on -fasm, because that halves compilation
time at the expense of a few percent performance. -Rghc-timing
prints out a line of timing info about each compilation. It's handy
to keep an eye on. -Wall turns on all the warnings; GHC is
meant to be warning-clean with -Wall.

GhcStage1HcOpts = -O -fasm

Build stage 1 optimised: we're going to be rebuilding stage 2 a lot,
so we want the compiler that does the building to be fast.

GhcStage2HcOpts = -O0 -DDEBUG -Wall

We turn off optimisation here, assuming you'll be modifying and
testing stage 2. With optimisation off, rebuilding GHC after
modifying it will be much quicker, not only because the
individual compilations will be quicker, but also there will be
fewer dependencies between modules, so much less stuff is recompiled
after each modification.

Also we turn on -DDEBUG, because that enables assertions and
debugging code in the compiler itself. Turning on DEBUG makes
the compiler about 30% slower.

GhcLibHcOpts = -O -fasm -XGenerics

You almost certainly want optimisation on when building
libraries, otherwise the code you build with this compiler
goes really slowly. -fgenerics add generics support to the
libraries - you can turn this off if you like (it'll make the
libraries a bit smaller), but you won't be able to use Generics in
the code you build against these libraries.

GhcLibWays = v

Normally the profiled libraries are built. Setting GhcLibWays to
just "v" disables this, so you only build the normal libs.

SplitObjs = NO

Object splitting causes each module to be split into smaller
pieces in the final library, to reduce executable sizes when
linking against the library. It can be quite time and
memory-consuming, so turn it off when you're hacking.

Building things

At this point you have made yourself a fully-configured build tree, so
you are ready to start building real things.

The first thing you need to know is that you must use GNU
make. On some systems (eg. FreeBSD) this is called
gmake, whereas on others it is the standard make command.
In this document we will always refer to it as make; please
substitute with gmake if your system requires it. If you use a
the wrong make you will get all sorts of error messages (but no
damage) because the GHC Makefiles use GNU make's
facilities extensively.

To just build the whole thing, cd to the top of your build tree and
type make. This will prepare the tree and build the various parts
in the correct order, resulting in a complete build of GHC that can
even be used directly from the tree (as inplace/bin/ghc-stage2),
without being installed first.

Order of the build

Here is a high level view of what happens when you build GHC:

First, we build a few packages that GHC itself depends on, such as
Cabal, and filepath, using your installed GHC. These packages
are under libraries, and each is built in a subdirectory
dist-boot; for example the bootstrap build of Cabal will be in
libraries/Cabal/dist-boot.

Then we build package ghc, still using the installed GHC.
The ghc package is a perfectly ordinary package. The source
code for the package is all the files in compiler/.

Now, still using the installed GHC, we build the ghc executable,
from source code in ghc/. This source code is just a small Haskell
program that depends on package ghc.

The resulting executable is
called the "stage 1" compiler (see
stages). You can run the
stage 1 compiler by invoking inplace/bin/ghc-stage1. The stage 1
build of GHC happens in compiler/stage1.

The stage 1 compiler is now used to build all the packages in the
libraries subdirectory, and the runtime system in rts.

Finally, the stage 1 compiler is used to build GHC itself again,
this time against the libraries we just built. This GHC is called
stage 2, and can be invoked as inplace/bin/ghc-stage2.

There's an optional final stage, using the stage 2 compiler to build a
stage 3 compiler, but this isn't strictly necessary, and is only used
to check that the stage 2 compiler is working properly.

What to do after make has finished

If make completes successfully, then it should have created
inplace/bin/ghc-stage2, which is a GHC binary you can run directly.
It supports all the usual features, including GHCi if you pass the
--interactive flag.

In fact, the inplace directory looks a lot like an installed copy of
GHC (see Building/Installing): there is a bin
subdirectory containing various programs that can be run, including
ghc-pkg, Haddock and hsc2hs.

You can now run the testsuite, see Building/RunningTests.
You can now install GHC, by typing make install.

What to do if you get a build failure

GHC is a complex system, with many platform-dependent components. We
try our best to make sure it builds out of the box as often as
possible, but build failures are not uncommon. If you get some kind
of failure, don't panic.

The chances are, someone else already encountered the same problem as
you and has reported it on a mailing list or as a ticket. Search for
the error message using your favourite search engine, and if that
doesn't turn anything up then search directly on this Trac. Finally,
if you don't find a solution:

Developing in a GHC build tree

This section describes how to make changes to the sources of GHC, or
its libraries and tools, and use the build system to build and test
those changes.

Bringing the whole tree up to date

The easy way to bring the tree up to date after making some changes is
simply to go to the top of the tree and say make. The build system
has enough dependencies encoded into it that this should rebuild
everything that needs to be rebuilt, in the correct order. The
downside is that it will build everything right though to the stage2
compiler and including all the documentation, which might be overkill
if all you wanted to do was to test a small change to GHC.

Building a single sub-component

Each subdirectory of the source tree has a
stub makefile,
most of which follow this pattern:

dir = libraries/base
TOP = ../..
include $(TOP)/mk/sub-makefile.mk

the main purpose of the stub makefile is to invoke make at the
top-level of the tree: GHC's build system is
non-recursive, so
in effect there is really just one Makefile, at the top level.
However, the stub makefile has another purpose: it passes a target to
the top-level Makefile, telling it to build just the components of
the system in this directory. For example, when you say make in the
rts directory, this is actually equivalent to

$ make -C .. all_rts

where "-C .." tells make to invoke the Makefile in the directory "..", and all_rts is the target that makes everything in the rts subdirectory.
Equivalently, make all_libraries/base at the top level would build
everything in the libraries/base subdirectory. To understand how
the all targets are defined, see
standard targets.

You can also clean a single component of the tree, just by saying
make clean in a subdirectory. Again this is equivalent to issuing a
command at the top of the tree of the form make clean_libraries/base.

Rebuilding the GHC binary after making changes

Suppose you want to make a small change to GHC itself and test it.
Assuming that your tree is already up to date, the best way to do this
is as follows:

$ cd ghc
$ make stage=2

Note that the first command above takes you to the ghc subdirectory of the source tree, not into the source tree (which is also named ghc if you did a git clone). So if you did a git clone from your home directory, you'll be in ~/ghc/ghc/, not ~/ghc/. Many of the compiler-building make commands must be performed from this subdirectory, not from the root of the source tree.

This will bring the stage 2 compiler up to date only. Setting stage=2 has the effect of disabling all the
rules that build the stage 1 compiler, so the build system will ignore the fact that the stage 1 compiler is also out of date, and hence all the libraries are also potentially out of date. If you just did make
from the top-level, all of these dependencies would be taken into
account, and a lot of rebuilding would probably ensue. There's another target
that takes an even quicker shortcut:

$ cd ghc
$ make 2

This is like make stage=2, except that it omits the dependency-building phase (make 2 is in fact just shorthand for make stage=2 FAST=YES; see Fast Rebuilding below). If you have changed the imports in any modules, those new dependencies will not be taken into account by the build system, so you might get a build failure. On the other hand, this shortcut usually works and the few seconds it saves can make GHC development a much more interactive experience. There are also targets

make 1

make 3

to make the stage 1 and stage 3 compilers respectively. These targets work in both the ghc and compiler subdirectories (Commentary/SourceTree).

Note that if you’ve never built stage3 before, you will need to create dependencies for it using make stage=3. This is because a normal build will skip building the stage3 compiler. You will then be able to run make 3 as usual.

Freezing stage 1

Often when working on GHC we find ourselves doing make 2 a lot. If we accidentally say make at some point, that will start building stage 1 (because presumably something in the GHC source code has changed), which has many knock-on effects: all the libraries will be reconfigured, rebuilt, and then stage 2 will be completely rebuilt. To prevent this from happening, we can "freeze" stage 1 by adding a line to mk/build.mk:

stage = 2

this prevents stage 1 from being rebuilt until this line is removed or commented-out again. It's a handy trick when you're working on GHC.

Building a single file

It's possible to tell make to build a single file, from any subdirectory in the tree. For example, suppose I want to build just the module Control.Monad in the base package, I can do it like this:

$ make libraries/base/dist-install/build/Control/Monad.o

(you have to know that dist-install is the distdir for a package, and object files are put in the subdirectory build). It's also possible to do this from the libraries/base subdirectory:

$ cd libraries/base
$ make dist-install/build/Control/Monad.o

suppose you wanted to build this module with a few extra flags, perhaps because you want to see what GHC's optimiser is doing on this module:

you could also cut-and-paste the command-line to add flags, but sometimes the EXTRA_HC_OPTS method is more convenient.

Fast rebuilding

Often when you're working in a particular part of the tree, the rest of the tree is up to date, and you just want to rebuild the component you're working on after making changes. To speed things up, we'd like to avoid having make check that everything else in the tree is up-to-date before building the component we're working on. So the GHC build system provides a couple of ways to do this:

make FAST=YES

Only has an effect in a subdirectory. Setting FAST=YES causes make to omit rebuilding the .depend file (if any),
and also omits some of the phases. FAST=YES is allowed in conjunction
with any other target; for example, it makes sense when rebuilding a single file, as in the previous section.

make fast

Shorthand for make all FAST=YES.

Another useful trick is

make stage=0

Does not build any GHC stages at all. stage=0 can be used in combination with other targets and settings.

Installing extra packages

The boot libraries are built as part of building GHC; they are built with the stage1 compiler, and imported when the stage2 compiler is compiled with stage1.

All other libraries are stand-alone Cabal packages, and the build system knows nothing about them. Nevertheless, it is common to want to install extra packages for the GHC in your build tree. Here are instructions for how to do so.